1. Field of the Invention
This invention is directed to a process for the production of 4,4'-methylene diphenyl diisocyanate and its associated oligomers in high yields and improved selectivity.
2. Description of the Prior Art
Organic isocyanates are materials of commerce employed in the production of polyurethane resins. Of increasing commercial importance are the polymethylene polyphenyl isocyanates, referred to as PMPPI. One member of the PMPPI family, 4,4'-methylene diphenyl diisocyanate, referred to in the industry as 4,4'-MDI, is especially desirable because it exhibits outstanding properties in polyurethanes.
The common process for preparing 4,4'-MDI and PMPPI is the phosgenation of the corresponding diamine (4,4'-MDA) and polyamine (PMPPA). The common process for preparing MDA and PMPPA is by the acid condensation of aniline and formaldehyde. Representative patents on this process and variations therein include U.S. Pat. No. 2,638,730, U.S. Pat. No. 2,818,433, U.S. Pat. No. 2,974,168, U.S. Pat. No. 3,476,806, U.S. Pat. No. 3,676,497, U. K. Pat. No. 1,277,740, U. K. Pat. No. 1,341,018, U. K. Pat. No. 1,365,190, Bel. Pat. No. 803,216, Bel. Pat. No. 815,840 and E. Germ. Pat. No. 109,615. In the presence of hydrochloric acid, formaldehyde condenses with aniline yielding a mixture of methylene bridged dimers (MDA), trimers (DMTA) and higher oligomers (PMPPA): ##STR1## The mechanism is believed to involve the intermediacy of the imine .phi.N.dbd.CH.sub.2 which, on protonation, yields a key carbonium ion intermediate ##STR2## Aniline is then alkylated with high para-selectivity to another intermediate, I. This intermediate (I) can then undergo proton ##STR3## addition at nitrogen giving the amino benzyl cation II, which, in turn, completes the condensation by adding to a second molecule of aniline: ##STR4## However, other processes can easily contribute, including a simple direct attack of the protonated formaldehyde (or some derivative), the direct counterpart to acetone-phenol condensation. The result is that the purity (selectivity) to the para-para isomer (4,4'-MDA) is not as high as desired. Further, the acidic conditions required in the typical condensation process present further problems. What is needed is a more selective, higher yielding process for the preparation of 4,4'-MDI and PMPPI.